1. Field of the Invention
The present invention relates to a spin valve sensor free layer structure with a cobalt based layer that promotes magnetic stability and high magnetoresistance.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a nonmagnetic gap layer at an air bearing surface (ABS) of the write head. The pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic field into the pole pieces that fringes across the gap between the pole pieces at the ABS. The fringe field writes information in the form of magnetic impressions in circular tracks on the rotating disk.
An exemplary high performance read head employs a spin valve sensor for sensing magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) which is an exposed surface of the sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is preferably parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in resistance of the spin valve sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers.
The sensitivity of the sensor is quantified as magnetoresistance or magnetoresistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. A spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor. The sensitivity of a spin valve sensor depends upon the response of the free layer to signal fields from a rotating magnetic disk. The magnetic moment of the free layer or free layer structure depends upon the material or materials employed for the free layer structure. As the magnetic moment of the free layer structure increases the responsiveness of the free layer structure decreases. This means that for a given field signal from the rotating magnetic disk the magnetic moment of the free layer structure will not rotate as far from its parallel position to the ABS which causes a reduction in signal output.
In order to improve the sensitivity of the spin valve sensor a soft magnetic material, such as nickel iron (NiFe), is employed. It has been found, however, that when the free layer structure employs a cobalt based layer in addition to the nickel iron (NiFe) layer that the magnetoresistive coefficient dr/R increases when the cobalt based layer is located between and interfaces the nickel iron (NiFe) layer and the copper (Cu) spacer layer. A cobalt based layer, such as cobalt (Co) or cobalt iron (CoFe), has a magnetic moment of approximately 1.7 times the magnetic moment of nickel iron (NiFe) for a given thickness. The addition of a cobalt or cobalt based layer increases the stiffness of the free layer structure in its response to field signals and reduces the sensitivity of the spin valve sensor. Further, the cobalt based material causes the free layer structure to have a hysteresis. This hysteresis is indicated in a hysteresis loop which is a graph of the magnetic moment M of the free layer structure in response to an applied field H (signal field) directed perpendicular to the easy axis of the free layer structure. The hysteresis loop, which is referred to as the hard axis loop, has an opening due to the hysteresis which can be on the order of 5 to 7 oersteds. The opening in the hard axis loop is quantified as hard axis coercivity HC which is measured from the origin of the x and y axes to the intersection of the loop with the x axis (applied signal). It has been found that when the hard axis coercivity is high the head generates Barkhausen noise which is due to the fact that the magnetic domains of the cobalt based layer are oriented in different directions. Accordingly, as the signal fields rotate the magnetic moment of the free layer structure some of the magnetic domains do not follow the directions of the signal fields. The magnetic domains that do not readily follow the signal field direction follow behind the signal field direction in an erratic behavior, referred to as jumps in their movements, which causes the aforementioned Barkhausen noise. This Barkhausen noise is superimposed upon the playback signal which is unacceptable.
In order to keep the hard axis coercivity at an acceptable low level, only very thin cobalt based layers can be employed, such as 2 xc3x85 thick. While a 2 xc3x85 thick cobalt based layer produces some improvement in the magnetoresistive coefficient dr/R, it has been found that thicker cobalt based layers will further increase the magnetoresistive coefficient dr/R. Considering all factors, including sense current shunting, a cobalt based layer on the order of 15 xc3x85 produces the highest magnetoresistive coefficient dr/R. Unfortunately, a cobalt based layer of this thickness causes the free layer structure to have a hard axis coercivity which unacceptably reduces the sensitivity of the read head to signal fields and produces Barkhausen noise. Accordingly, it would be desirable if cobalt based layers thicker than 2 xc3x85 could be employed in a free layer structure without the aforementioned problems of responsiveness to signal fields and the production of Barkhausen noise. If the hysteresis or opening in the hard axis loop could be eliminated the aforementioned moment versus applied field graph (M/H graph) of the responsiveness of the spin valve sensor would be simply a straight line. This straight line indicates that the read head will be magnetically stable upon the application of the signal fields.
Another factor affecting the magnetic stability, not involving Barkhausen noise, is the magnetic stability of the pinning layer. A typical pinning layer is nickel oxide (NiO) which pins a magnetic moment of the pinned layer structure. Nickel oxide (NiO) has a blocking temperature of about 220xc2x0 C. wherein the blocking temperature is the temperature at which all of the magnetic spins of the nickel oxide (NiO) pinning layer are free to move in response to an applied field. Unfortunately, there is a blocking temperature distribution wherein some of the magnetic spins of the nickel oxide (NiO) pinning layer are free to move at temperatures below 220xc2x0 C. The operating temperature in a magnetic disk drive is anywhere between 80xc2x0 C. to 120xc2x0 C. Should the read head be further heated due to striking an asperity on the rotating magnetic disk or be subjected to an electrostatic discharge (ESD) the temperature of the read head may rise sufficiently so that an unwanted magnetic field may rotate the pinned layer causing some of the magnetic spins of the nickel oxide pinning layer to rotate. When this occurs the nickel oxide (NiO) may not be sufficiently strong (exchange coupling field) to return the magnetic moment of the pinned layer to its original orientation perpendicular to the ABS. This will cause a loss of amplitude and increase asymmetry of the playback signals. Accordingly, it would be desirable if the blocking temperature distribution of the nickel oxide (NiO) and/or an alpha xcex1 iron oxide layer associated therewith could be narrowed so as to improve the magnetic stability of the pinning layer.
In a read head application the hard axis loop or curve of the free layer structure has to be determined after the free layer is subjected to annealing at a high temperature for a period of time. This is due to the fact that during the fabrication of a read/write head combination the aforementioned first, second and third insulation layers are baked photoresist. After spinning a photoresist layer onto a wafer substrate and patterning it, the photoresist layer is annealed at a temperature of approximately 220xc2x0 C. for a period of 6 hours. Accordingly, the hard axis loop or curve for a free layer structure in a read head that is combined with a write head has meaning only after this annealing.
I have found that by obliquely ion beam sputtering the cobalt based layer of the free layer structure that the hard axis coercivity can be reduced. I have further found that annealing during the hard back cycle of the insulation layers of the write head further reduces the hard axis coercivity to virtually zero. Ion beam sputtering is accomplished within a chamber which has a substrate where the spin valve sensor is to be fabricated and a target which has the material to be sputtered. An ion beam gun directs ionized gas onto the target which causes the target to sputter atoms of the material toward the substrate. In oblique ion beam sputter deposition the surface planes of the substrate and the target are at an angle with respect to one another. Accordingly, oblique ion beam sputter deposition improves the magnetic stability of a free layer structure having a cobalt based layer which is still further improved by annealing. In an exemplary embodiment the free layer structure may include a nickel iron (NiFe) layer which is sandwiched between first and second cobalt based layers wherein the first cobalt based layer interfaces the copper spacer layer as discussed hereinabove. The second cobalt based layer still further increases the magnetoresistive coefficient dr/R. It is preferred that all three of these layers be obliquely ion beam sputtered.
I have also employed the oblique ion beam sputter deposition process for improving the magnetic stability of a pinning layer structure that employs a first layer of nickel oxide (NiO) and a second layer of alpha a iron oxide. While in a preferred embodiment the oblique ion beam sputtering is employed for depositing both of these layers the sputtering may be employed for depositing only one of the layers. The result is that the blocking temperature distribution is decreased so that the pinning layer structure is more stable when the pinned layer is subjected to a field transverse to its pinned direction in the presence of heat.
An object of the present invention is to provide a spin valve sensor for a read head which has improved magnetic stability.
Another object is to provide a free layer structure with a cobalt or cobalt based layer in a spin valve sensor which promotes an increase in magnetoresistance with virtually no hard axis coercivity.
A further object is to provide a highly magnetically stable spin valve sensor that employs one or more cobalt or cobalt based layers in a free layer structure and a pinning layer structure which includes nickel oxide (NiO) and alpha xcex1 iron oxide layers.
Still a further object is to provide a spin valve sensor with a pinning layer structure of nickel oxide (NiO) and alpha a iron oxide that has an improved blocking temperature distribution.
Still another object is to provide various methods of making the aforementioned spin valve sensors.
Other objects and advantages of the invention will become apparent upon reading the following description taken together with the accompanying drawings.